KodeKloud Notes

Docker containers bundle everything an application needs to run—and nothing more—making it simple to move workloads across environments. In this guide, you’ll learn how to:

Define your app environment with a Dockerfile
Build a container image
Push the image to a registry (e.g., Docker Hub)
Run the image locally

1. Writing a Dockerfile

Start by creating a Dockerfile in your project root. This example shows a Spring Boot application running on OpenJDK 8 (Alpine):

# Use OpenJDK 8 on Alpine Linux as the base image
FROM openjdk:8-jdk-alpine

# Document the port the application listens on
EXPOSE 9999

# Build-time variable for the JAR file
ARG JAR_FILE=target/*.jar

# Copy the packaged JAR into the container
ADD ${JAR_FILE} app.jar

# Run the JAR file when the container starts
ENTRYPOINT ["java", "-jar", "/app.jar"]

Key directives:

FROM sets the base image.
EXPOSE documents which port the app listens on.
ARG defines a build-time variable for your JAR.
ADD copies your artifact into the image.
ENTRYPOINT specifies the startup command.

2. Building the Docker Image

Run this command from the directory containing your Dockerfile. Replace <docker-hub-username> and <image-name> as needed:

docker build -t <docker-hub-username>/<image-name>:v1 .

-t tags your image in the format repository/name:tag.

Build Cache Optimizations

Docker caches each layer by default. When you rebuild without changing earlier steps, subsequent builds are faster. To bypass the cache, add --no-cache.

3. Pushing to Docker Hub

First, authenticate using:

docker login

Then push your tagged image:

docker push <docker-hub-username>/<image-name>:v1

Authentication Required

You must be logged in to Docker Hub before pushing images.
Use docker login and enter your credentials when prompted.

4. Running the Container Locally

Map port 9999 on your host to port 9999 in the container:

docker run -d -p 9999:9999 <docker-hub-username>/<image-name>:v1

-d runs the container in detached mode.
-p host_port:container_port publishes container ports to the host.

Quick Reference: Docker CLI Commands

Command	Purpose	Example
docker build	Build an image from a Dockerfile	`docker build -t user/app:latest .`
docker push	Push an image to a registry	`docker push user/app:latest`
docker login	Authenticate with a Docker registry	`docker login`
docker run	Create and start a container from an image	`docker run -d -p 8080:8080 user/app:latest`
docker images	List all local images	`docker images`
docker ps	List running containers	`docker ps`

Links and References

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